WERC Publication Brief: Increasing white shark bites now leading cause of sea otter mortality in California. Updated August 2015.
This Brief Refers To:
Tinker, MT, BB Hatfield, MD Harris, JA Ames. 2015. Dramatic increase in sea otter mortality from white sharks in California. Marine Mammal Science. doi:10.1111/mms.12261
Southern sea otters (Enhydra lutris nereis) are not considered prey for white sharks (Carcharodon carcharias), but in California, sea otter carcasses have been increasingly observed with shark bite wounds over the past decade. USGS and California Department of Fish and Wildlife (CDFW) researchers have published the data behind this trend in the journal Marine Mammal Science.
Researchers examined 1,870 carcasses collected over 29 years in California shorelines and waters via the Southern Sea Otter Stranding Network led by USGS, CDFW, Monterey Bay Aquarium, and The Marine Mammal Center. Within the past 10 years, sea otter strandings with shark bites have increased exponentially—shark bites now account for more than half of all southern sea otter carcasses recovered in California. The Estero Bay and Pismo Beach regions have seen the greatest increase, followed by the northern Monterey Bay.
This marked change may be due to growing white shark populations or spatial shifts. Laws protecting white sharks may have led to their greater numbers in the northeast Pacific, although data on white shark abundance and trends are limited.
White sharks might also be spending more time feeding inshore, due to a boom in marine mammal populations, such as northern elephant seals (Mirounga angustirostris) at Piedras Blancas and Año Nuevo. An increase in shark bitten sea otters in winter and early spring may coincide with movements of sharks between feeding areas, particularly sub-adult sharks that are transitioning from a fish diet to a marine mammal diet.
Shark-bite evidence continues to point to white sharks alone, and not other shark species. Perplexingly, there remains no evidence that white sharks ever actually consume the sea otters they have bitten. Possible explanations for non-consumptive biting of sea otters by sharks include mistaken identity or investigatory bites, as well as lack of high caloric blubber or palatability of sea otters. This is the first-known case where white shark bites have come to represent a major source of mortality for a non-prey species.
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